With the aid of a sol-gel process it is possible to prepare inorganic-organic hybrid materials by controlled hydrolysis and condensation of alkoxides, of in particular silicon, aluminium, titanium and zirconium.
An inorganic network is built up by a sol-gel process. Organic groups can additionally be incorporated via correspondingly derivatized silicates, and can be utilized on the one hand for functionalization and on the other hand for the formation of defined organic polymer systems. Because of the large number of possible combinations both of the organic and of the inorganic components and because of the high capacity for influencing of the product properties by the preparation process, networks resulting from sol-gel processes typically offer a very wide range of variation. In particular, coating systems can be obtained by means of a sol-gel process and tailor-made to the most diverse profiles of requirements.
Compared with inorganic materials, the layers obtained by sol-gel processes are relatively soft. Even though the inorganic contents of the system provide a high level of crosslinking, the reason for the reduced layer hardness is believed to be due to the small size of the inorganic materials in the system, which reduces the mechanical properties (e.g., hardness and abrasion resistance) of the layers. By the use of filler-containing polymers, the favourable mechanical properties of the inorganic contents can be more fully utilized, since in this case the filler provides inorganic particles having sizes of several micrometers. However, the transparency of the materials is typically lost, and optical uses are typically not possible. The use of small particles of SiO2 (e.g., Aerosils® particles) for the preparation of transparent layers having increased abrasion resistance is possible, but at the low concentrations that are typically employed the abrasion resistance of the layers is unfortunately similar to those of the above-mentioned system. The upper limit of the amount of filler is determined in part by the high surface reactivity of the small particles, which results undesirably in the formation of agglomerations or in increases in viscosity.
DE 199 52 040 A1 discloses substrates with an abrasion-resistant diffusion barrier layer system, wherein the diffusion barrier layer system comprises a hard base layer based on hydrolysable epoxysilanes and a top layer arranged on top. The top layer is obtained by application of a coating sol of tetraethoxysilane (TEOS) and glycidyloxypropyl-trimethoxysilane (GPTS) and curing thereof at a temperature of <110° C. The coating sol is prepared by subjecting TEOS to pre-hydrolysis and condensation with ethanol as the solvent in HCl-acid aqueous solution. GPTS is then stirred into the TEOS prehydrolyzed in this way and the sol is stirred for 5 hours at 50° C. A disadvantage of the coating sol described in this publication is its low storage stability (pot life), as a consequence of which the coating sol must be further processed within a few days after its preparation. A disadvantage of the diffusion barrier layer systems described in this publication is furthermore that these have according to the Taber abrasion test which are unsatisfactory for use in automobile glazing.